In this study, the free-surface characteristics of a liquid Li wall jet for the Li target of the International Fusion Materials Irradiation Facility (IFMIF) are comprehensively reviewed. In developing the IFMIF Li target, a scientific understanding of the free-surface wave characteristics and the development of diagnostic tools to measure these characteristics were critical issues. The same issues must be faced in other liquid metal applications in fusion engineering, such as liquid first walls or liquid diverters. Thus far, diagnostic tools and methods to measure all of the characteristics of waves (i.e., wavelength, wave period, wave speed (free-surface speed), wave height (amplitude)), and average jet thickness have been developed, and the probability distributions applicable to these wave parameters, as well as their statistical characteristic values, have been determined, validating the stability of the IFMIF Li target. Our findings, both the wave characteristics and the diagnostic tools, can be appliedto not only the IFMIF Li target but also innovative liquid metal diverters or first walls in fusion engineering.